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Full text of "The Cambridge Natural History"

POSITION OF MARSUPIALS

in Myrmecobius (alone) is the third toe the longest ; and no great
difference can be detected between the third and fourth toes in
the case of the genera Phascologcde, DidelpJiys, and some others.
Professor Leche compares the predominance of the fourth toe
with the hyperphalangeal condition in the fourth toe of the embryo
Crocodile, and considers it an archaic feature, not surpassed by
the ancient characteristics of the Monotremata. Again it has been
pointed out that in Phascologcde and JPerameles, the epistropheus
(axis vertebra) has a separate rib as in Ornithorhynchus. In the
third place, the likeness of the teeth of Myrmecobius to those of
Ornithorhynchus is an argument in the same direction, which is
furthermore supported by the great age (Mesozoic) of the Meta-
therian group, if we are right in regarding those extinct creatures
- as Marsupials.
We may now mention certain facts which are not so generally
used. The partly primitive structure of the right auriculo-
ventricular valve in the Monotremata has no counterpart in
any Marsupial which has been dissected ; but there are traces
in the latter of the characteristic " ventral mesentery " of Orni-
thorJiynclius and JSchidna^ Mr. CaldwelFs interesting observation
upon the segmenting egg of the Marsupial, the incompleteness
of the first segmentation furrow (reminding us of the meroblastic
ovum of the Moiiotreme), may possibly not turn out to be so
exclusively Marsupial a feature as has been thought.
The balance of evidence thus points to the nearer relationship
of the Marsupials to the Eutherian mammals ; and their great
specialisation combined with certain evidences of degeneration
(disappearance in part of the milk dentition), and their age, point
to the fact that they are, at any rate, the descendants of an early
form of Eutherian. But they must have separated from the
Eutherian stock after it had acquired a definite diphyodonty and
the allantoic placenta, the two principal features of the Eutherian
as opposed to the Prototherian mammals.
Nevertheless it seems probable that the Marsupial tribe is
derived from some of the earliest Eutherians. And on this view
may be explained the retention of Prototherian characters.
The remaining Eutheria are obviously all to be referred to
one great division with the possible exception of the Whales,
whose affinities form one of the principal difficulties to the student
1 In Dendrolagus at any rate. See Proc. Zool* Soc* 1895, p. 132.